1. Technical Field
The invention relates generally to patterning an optical property on optical elements.
2. Background Art
The ability to pattern an optical property on an optical element is important for high performance optical products. One such property, birefringence, or double refraction, is a phenomenon that occurs in materials characterized by two indices of refraction. Typically, birefringent materials are optically anisotropic substances, examples of which are calcite and quartz. However, some optically isotropic materials, such as glass and plastic, become birefringent when subjected to stress. When a beam of light enters a birefringent material, the beam splits into two polarized rays traveling at different velocities, corresponding to two different angles of refraction. One ray, called an ordinary ray, is characterized by an index of refraction that is the same in all directions. The second ray, called an extraordinary ray, travels with different speeds in different directions and hence is characterized by an index of refraction that varies with the direction of propagation. If the light entering th e birefringent material is unpolarized or linearly polarized, the ordinary and extraordinary rays will have the same velocity along one direction, called the optic axis.
Controlling the pattern of birefringence in a given optical element is useful for many applications. For example, there recently has been an increasing need for signal carrying capacity of optical fibers used by the telecommunications industry. Traditional methods for boosting capacity of fiber communications lines have required either increasing the number of carrier fibers or increasing the transmission rates. Increasing the number of fibers is costly, while the transmission rate for communication systems is limited by existing technology. An alternative approach, based on the use of optical circulator devices, offers a way to double the communication capacity of each single fiber without upgrading equipment or adding new fibers. Application of optical circulators allows bidirectional, full-duplex communication on a single fiber. Polarization-insensitive optical circulators are finding increased use in a broad variety of applications, including optical amplifiers, optical add and drop systems, dense-wavelength-division-multiplexing (DWDM) networks, optical time domain reflectometers (OTDRs), and instrumentation. Polarization-insensitive optical circulators are passive devices that steer optical signal flow from port to port in one direction only, thereby preventing signals from propagating in unintended directions. Unlike xe2x80x9csplitters, xe2x80x9d which incrementally add losses for each splitter used, optical circulators are low-loss devices.
Birefringent waveplates are an integral part of many polarization-insensitive optical circulator devices. The birefringent waveplates that are used in polarization-insensitive optical circulators may be fabricated from the birefringent crystals which divide the light into an ordinary ray and an extraordinary ray. Birefringent glasses may also be used in forming birefringent waveplates as seen, for example, in U.S. Pat. Nos. 5,375,012 and 5,627,676, assigned to the assignee of the present invention. A waveplate introduces a phase shift between polarized components of light transmitted through the plate. The waveplate modifies and controls the relative phase of the ordinary and extraordinary ray of the beam. The phase differencexcfx86 between the two rays is given by:
xcfx86=+/xe2x88x92[2xcfx80l(nexe2x88x92no)/ xcex]
where ne is the refractive index of the extraordinary ray, no is the refractive index of the ordinary ray, l is the physical thickness of the plate and xcex is the wavelength of the light ray.
A simple waveplate may be a slice cut out of a uniaxial crystal, where the slice is cut so that the optic axis lies in a plane parallel to the face of the plane. Principally, materials such as quartz, mica, and calcite are used to form the waveplate. U.S. Pat. No. 5,375,012, issued to Borrelli et al. (the ""012 patent), discloses a waveplate composed of a transparent glass body having a thermally developed separated phase in the glass body, where the thermally developed phase is composed of amorphous or crystalline particles having a high aspect ratio. The particles are oriented and aligned along a common axis, whereby the glass body is rendered birefringent so that polarized components of light transmitted through the glass have a phase shift introduced. The waveplate disclosed in the Borrelli patent uses silver halide particles as the separated phase. Borrelli additionally discloses the use of lead borate glasses and bivalent metal oxide silicate glasses. U.S. Pat. No. 5,627,676, issued to Borrelli et al., discloses a waveplate similar to the example from the ""012 patent, but uses copper halide to generate the separate phase.
Orientation of birefringent half-waveplates is a critical feature in the performance of a polarization insensitive optical circulator. In the simplest possible arrangement, two half-waveplates are connected together, one half waveplate oriented to produce a positive light rotation and the other oriented to produce a negative light rotation. A half-waveplate may be defined as a plate of a proper thickness that introduces a phase difference of Π (or 180xc2x0) between the ordinary and extraordinary rays. The half-waveplates in the device must have different orientations to control the beam orientation, as shown in prior art FIGS. 1A and 1B. FIG. 1A shows a part of an optical circulator, where a rotator group R1 is composed of two reciprocal rotators (half-waveplates) disposed along the direction of light propagation. A first rotator QR1 is a composite rotator composed of reciprocal rotators QR11 and QR13 that rotate light clockwise by 45xc2x0 and reciprocal rotators QR12 and QR14 that rotate light counterclockwise by 45xc2x0, where QR1 installed in a plane vertical to the direction of the light. A second rotator is a reciprocal clockwise rotator QR2.
FIG. 1B shows a method by which the first rotator group QR1 is manufactured. In FIG. 1B, four separate half-waveplates (101, 102, 103, and 104) are assembled together, using an adhesive to generate the 2xc3x972 array (rotator group Q1) that is shown in FIG. 1A. In this example, blocks 101 and 104 have the same orientation (i.e. they are oriented to produce a rotation of +45xc2x0), while blocks 102 and 103 have the same orientation (i.e. they are oriented to produce a rotation of xe2x88x9245xc2x0). The difficulty, therefore, in manufacturing these elements arises from the fact that the blocks (101, 102, 103, 104) need to be placed individually and then combined to form the 2xc3x972 array (rotator group QR1). As the complexity of the polarization-insensitive optical circulator increases, the size of the array increases correspondingly to the number of ports in the polarization-insensitive optical circulator. Larger arrays, therefore, are difficult to manufacture using current manufacturing processes.
Polarization-insensitive optical circulators are available in a variety of configurations and performance options. The required number of ports, operating wavelength, polarization sensitivity, port isolation, and mechanical packaging are all variables that influence the choice of optical circulator. These designs, however, usually rely on the assembly of small individual birefringent elements cut from crystalline material. The assembly of these elements is non-trivial. The size of the parts which can be so assembled precludes miniaturization below a certain size because of the difficulty in handling the parts. Additionally, optical circulator designs have traditionally been limited by the shapes into which such individual crystal elements can be cut and assembled (typically stacked blocks). Ideally, the birefringent elements should eliminate the costly and complex assembly steps, allow further miniaturization, allow additional design freedom, and ease the manufacture of high-port-count circulators.
In one aspect, the invention relates to a method for patterning an optical property on an optical element by applying localized heating to the optical element. In another aspect, the invention relates to a method of patterning an optical property on an optical element by combining pieces of optical element containing the optical property with pieces of optical element without the optical property.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.